One application for power transistors employed in the prior art is use in an inverter to invert direct current electrical power to single or polyphase alternating current power. The power transistor performs such an inversion by switching the D.C. power in sequence to different alternating current output connections. Such a sequential switching operation is controlled by a logic circuit which supplies electrical pulses to the power transistors to render them conductive or nonconductive. Upon being driven into conduction by a signal from such a logic circuit, the transistor either conducts for a fraction of a cycle and is then turned off in a controlled manner to pulse width modulate the alternating current output or conducts with a 50% duty cycle and the D.C. voltage is varied (VVI).
Prior art power transistor driver circuits are designed with both forward and reverse biasing capability. A power circuit base drive circuit applies bias control for fast switching times under normal operation. The base drive circuit biases the power transistor in one biased state or the other, i.e. either forward or reverse biases the power transistor.
Optimization of such power circuits makes the power transistors susceptible to damage when exposed to overcurrent conditions. Such overcurrent conditions typically result in either an avalanche breakdown condition or punchthrough of one of the p-n junctions comprising the transistor.
In the event of an overcurrent, it is necessary to remove a forward bias and turn the transistor off and avoid either condition. In the prior art, a reverse bias is applied to turn off the transistor. Application of a reverse bias to the transistor's base input reduces or limits the transistors ability to withstand over-voltage conditions that accompany an overcurrent condition caused by transients and the like.
One prior art solution to this over-voltage problem is use of resistor/capacitor snubber circuits coupled across a power transistor. These snubber circuits limit the rate of change of the voltage across the power transistor. This solution, however, typically results in problems in mounting these components in the power circuits and results in additional cost as well as power losses within the circuit. Snubber circuits also reduce the turn-on capability of the power transistor and therefore slows response time of the power circuit.